COLLINS AND KEMPTON : A FIELD AUXANOMETER 207 



wooden shelf is supported. A narrow slot is cut from one end of the 

 shelf to within an inch or two of the other, and near the open end the 

 slot is enlarged to pass over the stake. A bolt provided with a wing 

 nut is passed edgewise through the board behind the opening for the 

 stake, permitting the shelf to be fastened at the desired height. The 

 auxanometer is attached to the shelf by means of a wood screw or bolt 

 which passes through the slot and into the bottom of the box. This 

 arrangement permits movement in either direction, and allows the 

 instrument to be brought directly over the plant to be measured. 



Satisfactory clocks can be had for $5.35 per dozen. The value of 

 all other materials is trifling and the cost of the finished machines, 

 including the labor, need not exceed $15.00 per dozen. 



Since the recording pen is attached directly to the growing part of the 

 plant by an inelastic rod, the accuracy of the measurements is not 

 affected by lack of precision in the construction of the instrument. 

 The only essential is that the parts move freely and with a minimum of 

 friction. If the axis of the recording drum and the glass rod are not 

 parallel, there will be a slight error in the absolute elongation recorded, 

 but this error will be constant throughout the record and will not affect 

 the comparative elongation of different periods. In this particular 

 this simple auxanometer may claim advantages in accuracy over the 

 more elaborate and expensive forms, in which the motion is transmitted 

 by a flexible thread and the direction of the motion is changed by 

 passing the thread over a revolving drum. With such instruments 

 it is difficult to eliminate slight errors due to hygroscopic changes in 

 the thread, and any inaccuracies in the curvature or centering of the 

 drum* are reflected in the measurements. 



In the making of continuous measurements of plants in the open, 

 the movement of the plant due to wind is always a disturbing element. 

 With the instrument here described the displacement of the plant, of 

 course, depresses the pen; but as the highest point that the pen can 

 reach at any given time is the position of rest, the effect of wind is to 

 cause a series of almost vertical lines always below the horizontal 

 line that marks the true elongation. In very gusty weather, when the 

 plants are unprotected, this may result in the formation of an almost 

 continuous band, but even then the upper margin of this band records 

 the correct elongation. The effect of a moderate wind is shown in the 

 last two hours of the record reproduced in figure 2. 



It is believed that the relatively high cost of the auxanometers that 

 are on the market, together with the fact that they are not adapted 



